SPANG, JOHN H.
Dept. of Geology and Geophysics and Center for Tectonophysics, Texas A&M University, College Station, TX

Abstract: New Style of Fault-Propagation Folding with Important Implications for Reservoir Geometry and Quality

In fault-propagation folds, the fault displacement goes to zero at the fault tip where it is consumed by folding at stratigraphic levels above the tip line. In the original model for fault-propagation folding, the foreland dipping limb of the anticline has the same dip as the hinterland verging anticlinal axial surface. Also, the foreland and hinterland verging anticlinal axial surfaces diverge at the stratigraphic level of the tip line forming a box-shaped anticline above and a single anticlinal axial surface below this stratigraphic level. The new model is more general and relaxes these restrictions, such that the forelimb of the anticline does not have the same dip as the hinterland verging anticlinal axial surface, and the two anticlinal axial surfaces do not intersect exactly at the stratigraphic level of the tip line. In the original model, all of the dip domains between the axial surfaces are either undeformed or have been deformed only once. On the forelimb of the anticline, the new model introduces an additional dip domain, which has been deformed twice in two different orientations. The dip domain, which has been deformed once, could maintain constant layer thickness, while the dip domain, which has been deformed twice, could change thickness, or both dip domains would change thickness by different amounts. In the case of a fractured reservoir, the dip domain that has been deformed twice might make a much better reservoir, and the orientation/timing of the fractures is known. Possible starting geometries include a fault-propagation fold that develops at the tip of a layer-parallel detachment, at the top of an initial dipping ramp, or above a planar fault in basement, and they produce different final fault/fold geometries. For example, a planar fault in basement creates a complex monocline versus box-shaped anticlines in the first two geometries. The new model is applied to Anschutz East Field (UT) and Sage Creek (WY).

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas